Optical fiber amplifiers (exemplarily comprising Er-doped silica-based optical fiber) are well known and play a crucial role in optical fiber communication systems. They are adapted for use in multi-wavelength systems with a multiplicity of signal channels at wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N. A "channel" herein generally is a wavelength slot in the signal band that carries signal radiation.
In multi-wavelength systems the number of channels may vary, exemplarily due to network re-configuration or channel failure. Thus, the steady state and transient channel addition/removal responses of optical fiber amplifiers are of concern in WDM systems. See, for instance, J. L. Zyskind et al., OFC '96, Vol. 2, San Jose, Calif., 1996, Postdeadline Paper PD 31-1. For instance, dropping one or more channels can give rise to error events in the surviving channels because the power in the channels may surpass the thresholds for non-linear effects such as Stimulated Brillouin scattering (SBS). Adding one or more channels to existing channels can depress the power of the existing channels below the receiver sensitivity. Thus, dropping or adding channels in prior art optically amplified multi-wavelength systems can lead to error bursts. Such error bursts are highly undesirable and will frequently be unacceptable to service providers. Consequently, optical fiber amplifiers may not be used in high capacity multi-wavelength optical communication systems unless the channel addition/removal response of the amplifiers can be controlled.
The prior art contains some approaches towards solution of the problem. These include fast automatic output power control which keeps the output power of the surviving channels constant by means of a feedback loop, as well as gain control. See, for instance, A. K. Srivastava et al., Technical Digest of the Optical Amplifiers and Their Applications 1996 Topical Meeting, OAA '96, postdeadline paper PDP4; and K. Motoshima et al, OFC '93, San Jose, Calif., 1993, pp. 40-42, Paper TuI5, respectively. These approaches are relatively complex and costly. For instance, the latter approach requires an additional laser. Another approach involves optical gain clamping. See, for instance, J. Massicott et al., Technical Digest, OAA '96, paper FB2, pp. 77-80. This approach requires some relatively expensive optical components (single wavelength optical reflectors) and typically increases the noise figure of the amplifier.
In view of the above facts and reasons and the shortcomings of prior art approaches, it is clear that it would be highly desirable to provide multi-wavelength optical fiber communication systems that are substantially free of the described shortcomings, and that achieve this at moderate cost and with little or no added complexity. Furthermore, there is in general a need for optical fiber amplifiers having low multi-channel gain variation, low gain cross saturation and a low multi-channel gain tilt. This application discloses such systems and such optical fiber amplifiers.